Introduction

This document answers frequently asked questions about wireless
systems, and covers areas such as antennas, polarization, interference, and
safety.

Q.
What type(s) of antennas can I use with my system?

A. Use any antenna that is:

Specified to work at the chosen or assigned carrier
frequency.

Specified to operate over at least the 6 or 12 MHz bandwidth, as
appropriate.

All antennas must have a 50-ohm impedance specification, and almost all
do. For the most part, your antenna choice(s) are based on gain and directivity
pattern characteristics required, which in turn are based on the range (path
length) of the link and the topology (point-to-point or multipoint).

Q.
Do the antennas for both ends of my link need to be the same exact size
or type?

A. No. For example, there are cases where the antenna-mounting
arrangements at one end of a link is only able to physically support relatively
small antennas, such as a one- or two-foot dish. Yet the link requires a larger
antenna at the other end to provide the necessary antenna gain for the path
length in question. Sometimes, a high-gain, narrow pattern antenna is necessary
at one end to avert an interference problem, which is probably not a concern at
the other end.

Remember that the total antenna gain for a link is commutative—if the
two antennas have different gains, you do not need to consider which antenna is
at which end (except in consideration of mounting/interference issues).

Warning: Even though the two antennas for a link can look very different from
each other, they must have the same polarization in order for the link to work
properly.

Q.
What is antenna gain? How does antenna gain relate to the pattern or
directivity?

A. The gain of any antenna is essentially a specification that quantifies
how well that antenna is able to direct the radiated radio frequency (RF)
energy into a particular direction. Thus, high-gain antennas direct energy more
narrowly and precisely, and low-gain antennas direct energy more broadly. With
dish-type antennas, for example, the operation is exactly analogous to the
operation of the reflector on a flashlight. The reflector concentrates the
output of the flashlight bulb into one predominant direction in order to
maximize the brightness of the light output. This principle applies equally to
any gain antenna, because there is always a trade-off between gain (brightness
in a particular direction) and beam width (narrowness of the beam). Therefore,
the gain and pattern of an antenna are fundamentally related. They are actually
the same thing. Higher gain antennas always have narrower beamwidths
(patterns), and low gain antennas always have wider beam widths.

Q.
What is antenna polarization?

A. Polarization is a physical phenomenon of radio signal propagation. In
general, any two antennas that are to form a link with each other must be set
for the same polarization. Typically, you set polarization through the way you
mount the antenna (or just the feedhorn). As such, polarization is almost
always adjustable at the time of antenna installation, or later.

There are two types of polarization, namely, linear and circular. Each
has two subcategories within: for , and right- or left-handed for .

Linear polarization is categorized as vertical or
horizontal.

Circular polarization is categorized as right-handed or
left-handed.

Polarization Category

Polarization Subcategory

Notes

Linear

Vertical or Horizontal

The vast majority of microwave or dish-type antennas are
linearly polarized.

Circular

Right Handed or Left Handed

Not encountered much in the commercial data communications
realm.

If, for example, the two antennas for a link are linearly polarized,
they must both be either vertically polarized or horizontally polarized. If
both antennas do not have the same polarization the link either works poorly or
does not work at all. The situation where one antenna is vertically polarized
and the other is horizontally polarized is known as cross-polarization.

For licensed links, the terms of the license can specifically dictate
the polarization. For unlicensed links, you are typically free to choose, and
the choice can be crucial to avert or correct an interference problem. See the
interference resolution section for more
information. Note that for most microwave (dish) antennas, you cannot determine
the exact type of polarization the antenna is set up for through observation
from a distance (such as when you view a tower-mounted antenna from the
ground).

Q.
What is cross-polarization?

A. When two antennas do not have the same polarization the condition is
called cross-polarization.

For example, if two antennas both had linear polarization, but one had
vertical polarization and the other had horizontal polarization, the antennas
are cross-polarized. The term cross-polarization (or "cross-pol") also
generally describes any two antennas with opposite polarization.

Cross-polarization is sometimes beneficial. An example of this is a
situation in which the antennas of link A are cross-polarized to the antennas
of link B, where links A and B are two different but nearby links that are not
meant to communicate with each other. In this case, the fact that links A and B
are cross-polarized is beneficial because the cross-polarization prevents or
reduces any possible interference between the links.

Q.
How can I tell if and when my antennas are properly aligned?

A. First of all, be sure that the two antennas for the link are not
cross-polarized. After that, you need to be sure that each antenna is pointed
or aligned to maximize the received signal level. A tool is commonly provided
on the radio equipment to help determine this, in the form of an indicator or
alignment
port (use the Find function on your browser to locate this term) for a
meter that gives a voltage reading proportional to the received signal level.
At one end of the link at a time, the antenna pointing direction is carefully
adjusted to maximize (or "peak") the reading on the indicator tool.

After this is done for both ends, you must obtain the actual received
signal level in dBm in order to verify that it is within 0 to 4 dB of the value
obtained from the link budget calculation. If the measured and calculated
values differ by more than about 8 dB, you can suspect either that the antenna
alignment is still not correct or that there is another defect in the
antenna/transmission line system (or both).

Note: You can get a "peak" reading during the antenna alignment process if
one or both of the antennas is aligned on a "side lobe," in which case the
measured receive level may be 20 dB (or more) lower than the calculated value
would indicate it should be. Be aware that the link may still work under these
circumstances. If you get agreement to within 0 to 4 dB between the measured
and calculated receive signal levels, you can be confident that the antennas
are properly aligned with no other problems.

Q.
The path for my link crosses through the path of another link. Will the
two links interfere with each other?

A. No. Any type of radio (or other electromagnetic) signal that propagates
through space (or air) remains unaffected by any other signal that happens to
cross the same point in space. In order to prove this, get two flashlights, and
shine one onto a wall. Hold the other flashlight a distance away from the
first, but point the second flashlight so that the two light beams cross. You
notice that the beam from the second flashlight has no effect on the spot on
the wall from the first. This same principle is true for radio signals of any
frequency. Of course, in the flashlight example, if you shine the second light
onto the same point on the wall, the spot appears brighter. If the beams were
radio signals of the same frequency, and the spot on the wall was a receive
antenna for one of the links, the second beam is indeed likely to cause
interference. However, this is a different situation from when the beams cross
in space.

Q.
The path for my link has some telephone and/or power wires that run
perpendicular through the path. Will these affect my link?

A. No. Problems are unlikely in this situation. At the radio frequencies
at which the links operate, the wires appear to be infinitely long conductors.
As such, there is bound to be some slight diffraction effect on the signal that
propagates across them. However, because the wires are thin, this effect is
very slight, so much so that you can not even measure the effect. There must be
no adverse impact on the operation of the link.

Q.
I notice that there is an unused coax cable already installed in my
building between where I want to install the wireless router interface and the
outdoor transverter. Can I just use this cable for the IF cable?

A. Probably not. First of all, the intermediate frequency (IF) cable (and
RF cable) must have a 50-ohm impedance specification. Some types of coax cables
that are/were used with LANs can have other impedance specifications, and thus
you cannot use such cables.

If you verify that the existing cable is a 50-ohm type, the cable still
must meet two other specification requirements before you can use the cable:

The total loss at 400 MHz for the entire run length must be 12 dB or
less.

The center conductor size of the coax must be #14 AWG or larger.

If these requirements are met, you can use the existing cable. If there
is any doubt, do not use the cable. Also remember that someone stopped using
the existing cable for a reason, and that reason can that the cable has some
invisible internal damage that caused the previous user expensive and
frustrating problems. Coaxial cable, and even its installation, is relatively
inexpensive, so do not take chances with your important link.

Q.
I am about to install an unlicensed link. Which antenna polarization must
I choose?

A. For your own single link, polarization does not really matter. However,
there are two situations in which polarization is important:

(a) There are other nearby links that you do not control.

(b) You plan to install, or have already installed, other links to
one of the end points of the new link.

For (a), determine whether the other nearby links are on a frequency
that can possibly cause you an interference problem. Then attempt to determine
the polarization of those links. If you can, you must set up your new link to
be cross-polarized to the nearby links.

For (b), the same applies as for (a), except that now you can easily
determine the frequency and polarization, because you deal with links that you
control. A site with multiple links is known as a hub, and any two links to
that hub that are on the same frequency (or a close enough frequency that they
could interfere with each other) must be cross-polarized to each other to avoid
potential interference problems.

Q.
I have just learned that the outdoor coax connections must be sealed, but
my link is already installed and operational. Is it too late to seal these
connections, and must I bother now?

A. You must seal the connections as soon as possible, as long as the
system is functional and has not yet suffered any moisture-related damage. Some
types of sealing products, such as Coax-Seal, enable you to seal the
connections without the need to disconnect the connections or take an
operational link off-line.

Q.
How much distance can there be, in miles, between the antennas at each
end of a link?

A. Unfortunately, this common question does not have a quick or simple
answer. Here are the factors that govern the maximum link distance:

Maximum available transmit power.

Receiver sensitivity.

Availability of an unobstructed path for the radio signal.

Maximum available gain for the antenna(s).

System losses (such as loss through coax cable runs, connectors, and
so forth).

Desired reliability level (availability) of link.

Some product literature or application tables quote figures, such as
"20 miles." In general, these quoted single values are optimum, with all of the
above variables optimized. Also, remember that the availability requirement has
a drastic affect on the maximum range. That is, the link distance can perhaps
be double, or more, than the quoted value if you are willing to accept
consistently higher error rates, which can be appropriate in an example where
you use the link only for digitized voice applications.

The best way to get a useful answer is to do a physical site survey,
which involves examination of the radio path environment (terrain and man-made
obstructions) at the proposed link location. The results of such a survey can
yield valuable information on:

The radio path loss.

Any issues that can further compromise link performance, for example,
potential interference.

When you obtain this information, you can choose and know the other
variables, such as antenna gain, and you can obtain a very definitive answer
for the maximum range.

Q.
What does the duplexer really do? Why must I order the correct, specific
one?

A. In short, the duplexer is a device that allows a transmitter and a
receiver to be connected simultaneously to the same antenna.

Any two-way wireless communication requires both a transmitter and a
receiver. If you want to transmit and receive at the same time (also known as
full-duplex operation), clearly the transmitter and
receiver must both operate at the same time. Even if each had its own antenna,
full-duplex operation can present a problem because the power output of the
transmitter is millions of times greater than the power level of signals the
receiver tries to receive. If these two devices operate at the same time in
close proximity (which they typically are), some of the energy from the
transmitter is bound to find its way into the receiver, where the energy is
more powerful in comparison to the signals the receiver wants to receive. When
the transmitter and receiver are connected to the same antenna, the problem
becomes even more acute.

In order for full-duplex to work at all, there has to be some scheme to
separate the transmit and receive signals. One common technique to do this,
which Cisco broadband wireless products employ, is to transmit and receive on
different frequencies. This system is called frequency-division duplex. The
idea is that the receiver will not be able to "hear" the transmitted signal
because the receiver is selective. The receiver only receives a frequency (or a
small range of frequencies) to which the receiver is tuned, and does not
receive the transmitted signal if the frequency is outside of the tuning range
of the receiver (called the receive passband).

Although this fundamental idea is quite sound, you can still face a
problem. The receiver obtains the selectivity characteristic through filters,
which pass certain frequencies and reject others. However, the types of filters
that are practical to incorporate into the internal circuitry design of the
receiver are not selective enough to prevent the relatively powerful transmit
signal from adversely affecting the operation of the receiver, even if the
transmit frequency is well outside the passband range of the receiver filter.
In this situation, add more filtering.

Think of the duplexer as just a pair of bandpass filters incorporated
together in one box. It has three connection ports:

The transmit (TX) port.

The receive (RX) port.

The antenna port.

The TX and RX ports are usually interchangeable. In most
implementations (including Cisco's broadband wireless solutions), the duplexer
is a passive device. The duplexer neither requires nor consumes any power.
Consequently, you cannot configure the duplexer, either through software
control or other means.

In fact, some mechanical adjustments are made at the time of
manufacture, but after that time there must never be any need to readjust
these, and so any adjustment or calibration access points are typically sealed
and you must not tamper with them. The two passband filters that make up the
duplexer are very steep-skirted, which means they easily pass frequencies
within the passband, but then greatly attenuate signals that are outside of the
passband frequency range by only a small amount. This characteristic is
important to enable the duplexer to keep powerful transmit signals out of the
receiver. The requirements of steep-skirted selectivity and high out-of-band
attenuation are what make the duplexer unique. The duplexer must also be able
to handle the power level of the transmitted signal that passes through.

The duplexer has two non-overlapping passband frequency ranges, and
thus one is naturally higher than the other. You can set up a system to
transmit through the higher frequency passband filter and receive through the
lower frequency one, or vice-versa. These two scenarios are usually described
as transmit-high or transmit-low. The duplexer is not concerned with how this
is done. The only real requirement, as far as the duplexer is concerned, is to
make sure that the transmit frequency falls within the passband range of one of
the filters of the duplexer, and the receive frequency falls within the other.
This requires that you know the passband frequency ranges of the duplexer, and
the TX and RX operating frequencies when you install or operate the duplexer.

In practice, you must first determine, to at least some rough degree,
what the transmit and receive frequencies must be. Then, choose a duplexer with
appropriate TX and RX passband ranges to accommodate the necessary operation
frequencies. This does not require an infinite range of offerings of duplexers.
Rather, they are provided in a relatively few choices, one of which fulfills
the requirement. If you try to operate on a TX or RX frequency (or both) that
falls outside of the passband range(s) of the duplexer, the system does not
work. After you install or order the system, if you want to alter either the TX
or RX frequencies (or both), you can do so as long as any new frequencies that
you choose fall within the passbands of the duplexer. Otherwise, you must
obtain a different duplexer (for each end of the link).

Finally, note that you cannot reverse the existing TX/RX split (change
TX high to TX low, or vice-versa) unless you also physically reverse the
connections to the duplexer. Otherwise, the system cannot work after the split
is reversed in the setup configuration, because now neither the TX nor RX
frequencies fall within the duplexer passbands. For the Cisco Systems solution,
in order to reverse the duplexer connections, you must remove the duplexer from
the transverter, "flip" it around, and re-install it.

Q.
Are there any safety concerns regarding antennas or the radio system in
general?

A. Yes. Aside from the obvious concerns, such as safety when you climb
structures or when you work with dangerous AC line voltage, you must also be
aware of the issue of exposure to RF radiation.

There is still a lot that is unknown, so there is much debate about the
safe limits of human exposure to RF radiation.

Remember that the use of the word "radiation" here does not necessarily
connote any linkage to or issue with nuclear fission or other radioactive
processes.

The best general rule is to avoid unnecessary exposure to radiated RF
energy. Do not stand in front of, or in close proximity to, any antenna that
radiates a transmitted signal. Antennas that are only used to receive signals
do not pose any danger or problem. For dish-type antennas, you can safely be
near an operating transmit antenna if you are to the back or sides of the
antenna, because these antennas are directional and potentially hazardous
emission levels are only present at the front of the antenna. For more details,
refer to the
radiation
hazard calculation table. Use the Find function on your browser to
locate this term.

Always assume that any antenna transmits RF energy, especially because
most antennas are used in duplex systems. Be particularly wary of small-sized
dishes (one foot or less), because these dish antennas often radiate RF energy
in the tens-of-gigahertz frequency range. As a general rule, the higher the
frequency, the more potentially hazardous the radiation. If you look into the
open (unterminated) end of waveguide that carries RF energy at 10 or more GHz,
you can suffer from retinal damage if the exposure lasts only tens of seconds
and the transmit power level is only a few watts. There is no known danger if
you look at the unterminated end of coaxial cables that carry such energy. In
any case, be careful to ensure that the transmitter is not operational before
you remove or replace any antenna connections.

If you are on a rooftop and near an installation of microwave antennas,
do not walk, and especially do not stand, in front of any of the equipment. If
you must traverse a path in front of any such antennas, there is typically a
very low safety concern if you move briskly across an antenna's path axis.

Q.
How do I know if I need the diversity option? If I do need it, what kind
of antenna must I use?

A. In general, the diversity option is not necessary if the link is
unobstructed. In other words, you do not require the diversity option if the
link is a "radio line-of-sight" link.

The diversity feature of Cisco's broadband wireless solutions is
designed to allow reliable link operation in installations where you cannot
achieve line-of-sight, and where establishment of a usable radio link would not
be possible otherwise. The diversity transverter, when installed, is used only
to receive signals. The diversity transverter does not transmit.

Note that the diversity option is not effective if the obstruction to
the path is severe, for example, obstruction due to a mountain. The option is
most effective in urban installations where the path might be line-of-sight
except for one or two buildings in the path, for instance. In such cases, the
best way to know the degree of effective performance gain that the diversity
option provides is the empirical approach—install and see.

There is a way to run a test on an installed non-diversity link to get
a fairly good idea of how much such a link can benefit from the addition of the
diversity feature. Refer to the wireless line card documentation for
information about
throughput
setting. Use the find function on your browser to locate this
term.

In general, the antenna of the diversity transverter must be the same
as the antenna you use for the main transverter, but this is not an absolute
requirement. However, the polarization of the diversity antenna must be the
same as the main antenna.

Q.
Is there any way to know how likely I am to experience an interference
problem?

A. When you consider the possibility of interference problems, there are
some "common sense" items to know and watch out for. Here is the list:

Understand that operation in unlicensed bands carries an inherently
higher risk of interference, because the controls and protections of a license
are not afforded to you.

In the United States, for example, the Federal Communications
Commission (FCC) does not have any rule that specifically prohibits a new user
from installing a new unlicensed-band radio link in your area and on "your"
frequency. In such a case, you can experience interference. However, there are
two issues to consider in such a situation.

If someone installs a link that interferes with you, chances are
that you also interfere with them. The other party can note the problem during
system installation, and choose another frequency or channel.

With point-to-point links that employ directional antennas, any
signal source (of a comparable power level to yours) that can cause you any
interference would have to be closely aligned along your own path axis. The
higher the gain of the antennas you use, the more precisely the interfering
signal would have to be aligned with your path in order to cause a problem.
That is why, Cisco recommends that you use the highest-gain antennas for
point-to-point links as is practical. Thus, in unlicensed bands, the potential
for interference from another unlicensed user, as a practical matter, is not
much greater than for licensed bands, where you essentially "own" your
frequency.

Remember that some licensed users sometimes operate in the unlicensed
bands as well. The unlicensed bands are allocated on a shared basis, and while
there is no requirement for you to obtain a license to operate for low-power
datacom applications with approved equipment, other licensed users can be
allowed to operate with significantly higher power. A specifically important
example of this is operation of U.S. government radar equipment in the U.S.
U-NII band at 5.725 to 5.825 GHz. These radars often operate at peak power
levels of millions of watts, which can cause significant interference problems
to other nearby users in this band. Therefore, look around your site to
determine whether there are any airports or military bases, where such radars
can exist. If so, you must be prepared to experience periods of interference.

If you are a licensed user and you operate in a licensed band, you do
not have to worry about interference. If you experience problems, there are
legal statutes that provide for resolution of the matter.